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IGNITION SYSTEM REQUIREMENTS AND THEIR 

APPLICATION TO THE DESIGN OF CAPACITOR 

DISCHARGE IGNITION SYSTEMS 



Terrence Lyle WMliamson 



NAVAL POSTGRADUATE SCHOOL 

Monterey, California 




THESIS 



IGNITION SYSTEM REQUIREMENTS AND THEIR 

APPLICATION TO THE DESIGN OF CAPACITOR 

DISCHARGE IGNITION SYSTEMS 



by 



Terrence Lyle Williamson 



Thesis Advisor; 



R. W, Adler 



December 1971 



Appnx)VQ,d (^on. pubtic A.cXea6e; dUtAibatLon unlimitzd. 



Ignition System Requirements and Their Application 

to the 
Design of Capacitor Discharge Ignition Systems 



by 



Terrence Lyle Williamson 
Lieutenant, United States Naval Reserve 
B. S. , Weber State College, 1965 



Submitted in partial fulfillment of the 
requirements for the degree of 



MASTER OF SCIENCE IN ELECTRICAL ENGINEERING 



from the 

NAVAL POSTGRADUATE SCHOOL 
December 1971 



Ti^> < '-• / -^ 



ABSTRACT 

Kettering ignition systems used on the majority of automotive engines can 
no longer assure reliable ignition for high-output engines. The capacitor dis- 
charge ignition, CDI, is a promising system to supersede the obsolete battery- 
coil. 

This study examines wave-front requirements at the spark plug for pro- 
ducing ignition in the internal combustion engine and system characteristics 
necessary for producing the wave-front. Arc requirements are described and 
used to define CDI parameters. 

A modified Kettering system which violates some basic ignition concepts 
was replaced by a CDI system designed in this study. Its characteristics were 
derived from arc requirements, not by aggrandizing the replaced battery-coil 
parameters. 

During performance tests, the CDI system exhibited superior performance. 
It fired simulated fouled plugs and continued to produce an arc when pressurized 
tx) 3 times the value at which the Kettering ceased to function. This improved 
performance was accomplished with approximately the same stored energy 
and less input power. 



TABLE OF CONTENTS 

L INTRODUCTION 8 

n. IGNITION SYSTEM PARAMETERS 9 

A. SPARK PLUGS 9 

1. Fouling 9 

2. Fouled Plug Simulation 11 

B. SPARK PARAMETERS . 11 

1. Gap Width 11 

2. High Tension Voltage Requirements 13 

a. Compression 13 

b. Gap Spacing 14 

c. Electrode Temperature 14 

d. Speed and Load 14 

e. Acceleration 16 

f. Ignition Timing 16 

g. Fuel-Air Ratio 16 

h. Voltage Polarity 16 

i. Electrode Condition 18 

j. Overview of Voltage Requirements 18 

3. Spark Duration 19 

4. Voltage Rise Time 20 

5. Energy Requirements 23 



m. IGNITION SYSTEMS 25 

A. KETTERING IGNITION SYSTEM 25 

B. HITACHI IGNITION SYSTEM 29 

C. PIEZOELECTRIC IGNITION 31 

D. TRANSISTORIZED IGNITION SYSTEM — — — 32 

E. CAPACITOR DISCHARGE IGNITION SYSTEM 33 

1. Operation 35 

2. CDI, Improved Ignition Characteristics 36 

IV. IGNITION SYSTEM DESIGN REQUIREMENTS 39 

A. BRUTE FORCE CRITERIA 39 

B. GENERAL DESIGN REQUIREMENTS 40 

C. SPECIFIC REQUIREMENTS FOR A SYSTEM WHICH IS TO 
REPLACE THE HITACHI IGNITION SYSTEM 42 

1. Maximum Voltage Requirements 42 

2. Spark Plug Gap 42 

3. Spark Duration 42 

4. Storage Capacitor And Energy Requirements 42 

5. Rise Time 43 

6. Power Input Requirements 43 

7. Sunmiary of Design Criteria 44 

V. FEASIBILITY STUDY 45 

A. HITACHI IGNITION EVALUATION 45 

B. CDI SIMULATION 45 

C. CONCLUSION — 48 



VL GDI SYSTEM DESIGN 51 

A. DG TO DG GONVERTER 51 

B. DISGHARGE GIRGUIT AND MV GATE 55 

G. SGR TRIGGER GIRGUIT 57 

Vn. GDI SYSTEM BENGH TEST AND EVALUATION 62 

VniXONGLUSIONS AND REGOMMENDATIONS 77 



LIST OF ILLUSTRATIONS 
Figure 

1. SPARK WAVEFORM • 12 

2. EFFECT OF COMPRESSION PRESSURE ON VOLTAGE 
REQUIREMENTS 15 

3. EFFECT OF ELECTRODE TEMPERATURE ON VOLTAGE 
REQUIREMENTS 15 

4. EFFECT OF ENGINE SPEED ON VXTAGE REQUIREMENTS 15 

5. EFFECT OF ACCELERATION ON VOLTAGE REQUIREMENTS 17 

6. EFFECT OF SPARK TIMING ON VOLTAGE REQUIREMENTS 17 

7. EFFECT OF FUEL-AIR RATIO ON VOLTAGE REQUIREMENTS— 17 

8. EFFECT OF IGNITION TIMING AND SPARK DURATION ON 

ENGINE OUTPUT 21 

9. ENERGY VERSES CAPACITANCE 21 

10. KETTERING IGNITION SYSTEM 26 

11. KETTERING IGNITION 26 

12. HITACHI IGNITION SYSTEM 30 

13. TRANSISTOR IGNITION SYSTEM 30 

14. CDI BLOCK DIAGRAM 34 

15. HITACHI MODEL 46 

16. LISA SOLUTION FOR HITACHI MODEL 47 

17. CDI MODEL 49 

18. COMPUTER OUTPUT OF CDI SYSTEM 50 



19. GDI BLOCK DIAGRAM 52 

20. DC-TO-DC CONVERTER 54 

21. DISCHARGE CIRCUIT AND MV GATE 56 

22. UJT TRIGGER 59 

23. FORWARD GATE CHARACTERISTICS 60 

24. TURN-ON TIME CHARACTERISTICS 60 

25. GDI SCHEMATIC DIAGRAM 61 

26. HITACHI OUTPUT 65 

27. HITACHI OUTPUT PHOTOGRAPHS 66-68 

28. CDI OUTPUT 69 

29. CDI OUTPUT PHOTOGRAPHS 70-71 

30. SYSTEM WAVEFORMS 72-75 

31. CDI SYSTEM POWER CONSUMPTION 76 



I. INTRODUCTION 

The ignition system currently used on the majority of automotive engines 
has been improved only slightly since its introduction in 1914 by Charles F. 
Kettering. This pioneer system, referred to as the battery-coil or Kettering 
ignition is incapable of keeping pace with the demands put on it by todays engines. 

This study deals with the requirements necessary to produce ignition of a 
fuel-air mixture in an automotive engine combustion chamber. Ignition require- 
ments are divided into arc characteristics for proper ignition and system require- 
ments to produce this arc. 

To evaluate the processes used to produce the arc, various ignition systems 
are discussed. Their basic operation and characteristics are described to give 
insight into ignition system requirements. 

A specific, modified battery- coil ignition system is described and its char- 
acteristics listed. This system is described in detail since a capacitor discharge 
ignition, GDI, system is designed to replace it. Characteristics of both systems 
were evaluated and the improved operating characteristics of the CDI system 
noted. 

Ignition requirements are established and are applicable to new ignition system 
principles. These requirements are applied to the design of the system. 

This study concludes by recommending the implementation of CDI as the 
standard ignition for modern automotive engines. 



IL IGNITION SYSTEM PARAMETERS 

Abundant literature exists on ignition systems and on the spark character- 
istics necessary to assure ignition of the explosive charge in the combustion 
chamber. The difficulty arises in ferreting out what requirements are necessary 
and applicable; the sources of information are not all in agreement on just what 
parameters should be considered in the design of an ignition system. 

This section defines ignition parameters and their relationship to proper 
ignition. 

A. SPARK PLUGS 

The spark plug is that portion of the ignition system producing the arc that 
ignites the fuel-air mixture. If the arc does not have proper characteristics, 
ignition will not take place and misfiring will result. It is the duty of the ignition 
system to supply the necessary voltage and energy to the spark plug. 

Also, the spark plug is the common element in all ignition systems. Regard- 
less of the driving source configuration, the plug produces ignition by an arc 
occuring between electrodes. Gap configurations may vary, but arc generation 
and its characteristics remain basically unchanged. 

1. Fouling 

Fouling can be attributed to metallic compounds found in combustion 
deposits. These materials, accumulating on the insulator firing end, become 
electrically conductive, under certain operating conditions, and can thus prevent 
the ignition voltage from building up sufficiently to fire the plug. 



Fouling is caused by many factors such as: engine make and model, 
engine power utilization, spark plug design and heat range, anti-knock additives 
and other fuel additives, and oil consumption. It occurs due to accumulation of 
deposits under low temperature (low output) or high temperature (high output) 
conditions. 

Dry, fluffy black carbon deposits result from overrich carburetion, 
excessive choking, or a sticking manifold heat valve. Low ignition output can 
reduce voltage and cause misfiring. Excessive idling and slow speeds under 
light load also can keep spark plug temperature so low that normal combustion 
deposits are not burned off. 

Deposits accumulating on the insulator are by products of combustion 
and come from the fuel and lubricating oil, both of which today generally contain 
additives. Most powdery deposits have no adverse effect on spark plug opera- 
tion; however, they may cause intermittent missing under severe operation 
conditions, especially at high speed and heavy load. Under these conditions 
the powdery deposits melt and form a shiny yellow glaze coating on the insulator 
which, when hot, acts as an electrical conductor. This allows the current to 
follow the deposits instead of jumping the gap. 

The average driver cannot operate in a range that will best prevent 
fouling. He is likely to be subjected to both types of fouling since he will drive 
under low output while in city traffic yet in a high output situation on the express- 
ways. This requires the ignition system to fire a fouled plug under all engine 
operating conditions. 



10 



2. Fouled Plug Simulation 

As discussed above, a fouled plug will present a conductive shunt 

pjath for the ignition current. A high voltage noninductive resistor connected 

2 3 
from the spark plug to ground may be used to simulate a fxiled plug. * This 

resistance is usually in the range of 0. 5 to 1. Mi\. The 1 Ma. test is 

intended to simulate system performance with a fouled plug and is an industry 

standard test. 

B. SPARK PARAMETERS 

To ignite the fuel-air charge in an internal combustion engine, the spark 
must meet certain criteria. The basic parameters effecting the spark are gap 
width, high tension voltage, spark duration, rise time, and energy. 

Figure 1 is the waveform observed on an oscilloscope placed across the 
gap of a spark plug. Areas 1 and 3 iqjresert the ignition voltage rise time and 
arc sustaining voltage duration respectively. The arc is struck at point 2 and 
extinguished at point 4. 

1. Gap Width ^'^'^*^ 

Cycle-to-cycle variations in ignition consistency is related to the 
condition of the gap. The plug location and purging of the gap are important. 

The gap must have a minimum spacing to enable the arc to transfer 
adequate heat energy to the fuel-air mixture. The mixture has a natural tendency 
to quench or cool everything within its path. Wetted spark plug electrodes pro- 
duce boundary layers of fuel-air ratios too rich to ignite. 

A wide gap will enhance the circulation of mixtures of ignitable 
ratios within the gap area. A lean mixture, with greater molecular spacing of 

11 




> 
o 
5 



o 
a. 

CO 



12 



fuel and air particles, requires a wider gap in order to allow adequate quantities 
of the mixture within the proximity of the spark for the necessary heat transfer to 
Initiate combustion. 

Widening the gap increases the capacitive energy delivered to the gap. 
This is a result of the increased voltage required to initiate the arc between the 
electrodes. Since E = |CV , the additional energy available will be proportional 
to the square of the additional voltage required. 

The gap is typically set at the minimum value that provides smooth 
engine idle. Basically, gap width should be as large as possible but not so large 
that the ignition harness and distribution system will not handle the voltage required 
to create ionization. Allowances must also be included for gap growth. 

In the past, gap widths have been limited by available ignition voltages. 
This problem has been diminished and large gap widths, up to 0. 050 in. , may 
now be recommended. The limiting factor now is the voltage breakdown of the 
ignition system components. The gap width should be limited so that the maximum 
voltage required under the worst conditions is approximately 22 kV. 
2, High Tension Voltage Requirements 

The voltage required to cause arc-over is dependent on engine design 

and operating conditions as well as spark plug geometry. Variations from 4 to 

3 6 7 8 9 
20 kV. for various engine operating conditions are not uncommon. » » » ♦ 

a. Compression 

The variable which is usually considered first, due to the tradi- 
tional method of bench testing, is compression pressure. While the absolute 
value of sparking voltages will vary somewhat depending on the type of fuel. 



13 



moisture content and voltage source, this is basically a linear relationship, 
as indicated in Fig, 2, with the voltage required increasing as pressure sur- 
rounding the gap increases. However, because of the nonuniform electric 
field gradient within the plug gap, the breakdown voltage does not follow 
Paschen's Law exactly. 

In general, Paschen's Law states that the voltage required 
to jump a given gap in a uniform field is dependent only upon the product of the 
gas pressure and the electrode spacing. Increased gap size or pressure results 
in less breakdown voltage than predicted by Paschen's Law. 

b. Gap Spacing 

Other factors being equal, sparking voltages will increase 
directly with gap spacing within the normal range of usable settings as shown 
in Fig, 3, The discussion on Paschen's Law above also applies here. 

c. Electrode Temperature 

Temperature has a marked effect on voltage requirements. 
The lower the temperature, the higher the voltage required to cause arc over. 
This effect is also shown in Fig, 3. 

d. Speed and Load 

The effects of speed and load in a typical 4-cyle automotive 
engine are illustrated in Fig. 4, The slight decrease noted at high speeds can 
be attributed to increased spark plug electrode temperatures and decreased 
compression pressures which occur as the engine's breathing efficiency decreases. 



14 



Voltage 
Required 



Fig. 2. 



Compression Pressure 



Voltage 
Required 



Voltage 
Required 




Electrode Temperature 



Wide Open 
J Throttle 




Roa d Load 



Speed 



Fig. 3 



Fig. 4 



15 



e. Acceleration 

Sudden, wide-open-throttle acceleration causes rapid but 
temporary rises in voltage requirements as shown in Fig. 5. This increase is 
attributed to the rapid increase in pressure. The effect here is greater than 
that due to temperature, since the spark plug electrodes have not had time to 
heat up. 

These sudden voltage. increases are transient in nature and 
explain why misfiring is often encountered first during periods of rapid accel- 
eration, 

f. Ignition Timing 

The typical effect of spark advance on voltage is illustrated 
in Fig. 6. Advancing ignition timing lowers voltage requirements because the 
spark plug fires at a lower pressure, and the electrodes are hotter because of 
less charge cooling and higher flame temperatures. 

If the spark is retarded past top dead center, requirements 
decrease as compression at the point of ignition drops, and power and temper- 
ature are reduced. 

g. Fuel-Air Ratio 

Lowest voltage requirements will be observed at the stoichi- 
ometric ratio as shown in Fig. 7. Leaning of the fuel charge has the greatest 
effect on voltage requirements, although the overall effects can be considered 
negligible in the normal range of fuel-air ratios. 
h. Voltage Polarity 

Voltage polarity, commonly called coil polarity, is an often 
overlooked yet important factor. It must be considered because on all conventional 

16 



Voltage 
Required 




Acceleration 



Fig. 5 



Speed 



T. D.C. 



Voltage 
Required 



Fig. 6 



Adv. Ret. 

Spark Timing 



Voltage 
Required 



066 



Fig. 7 



Lean Rich 

F/A 



17 



spark plug designs, the center electrode operates considerably hotter than the 
ground electrode. Electron theory states that electrons move more readily from 
the hot to the cold electrode than the inverse. Therefore, the voltage applied to a 
plug must rise in the negative direction in order to produce ionization at minimimi 
voltage. 

If the plug has reverse polarity with respect to that defined 
above, the firing voltage is greater. In some instances, the difference may be 
a few thousand volts. 

i. Electrode Condition 

Sharp or pointed electrodes concentrate the gap ionization 
by increasing the electric field gradient. Therefore, spark plugs can be expected 
to require progressively greater voltage as the sharp corners of the electrodes 
erode away and become rounded in normal service. 

Fouling deposits do not influence the arc-over voltage, unless 
the deposits are within the gap area, which is seldom the case. 

j. Overview of Voltage Requirements 

The maximum available voltage from the coil should not 
exceed 30 kV. as any voltage higher than this can produce undue strain on the 
ignition harness, distributor, and spark plugs. At any voltage exceeding 30 kV. 
the spark plug insulation can flashover either internally or externally, and 
similiar flashovers can occur within the distributor either from the cover 
electrodes to ground or between electrodes. These flashovers can form carbon 
paths that once started can seriously down-grade engine performance. 



18 



Manufactures of electronic ignition systems have advertised 
voltages as high as 60 kV. , pursuing the theory that if some is good, more is 
better and the more voltage, the better the system. Actually, under worst 
conditions, an engine should not require more than 22 kV. across the plug to 
establish ionization. Under normal conditions, the peak voltage is consider- 
ably lower than this. The objective of an ideal system would be to produce the 
required voltage at any engine speed while maintaining the proper energy for 
the required length of time. 
3. Spark Duration 

Optimum spark duration for ignition of the fuel-air mixture has not 
been determined. Values range from a microsecond to thousands of micro- 
seconds. 

At present, spark duration requirements are evaluated on test 
engines where the duration that provides the highest engine output is considered 
optimum. This procedure does not demarcate the requirements for ignition of 
the combustible mixture, but rather the deficiencies that exist in combustion 
chamber design. 

For ignition to take place there must be a combustible mixture 
between the spark gap. Longer spark durations have a higher probability of 
igniting a mixture that is not homogenous throughout the combustion chamber. 
Long arc durations give sufficient time to permit the fuel charge to come within 
the gap area. 

Ignition timing has a large control over the turbulences that exist 
in the combustion chamber. Figure 8 illustrates the effect of ignition timing 



19 



and spark duration on engine output. Analyzing Rg. 8 more closely, it is seen 
that spark duration has limited effect on engine output if the specified engine 
timing is used, therefore, overadvanced timing is avoided. 

Practically all of todays automotive engines use the battery-coil 
ignition system with spark durations typically 1,000 to 2,000 usee. Current 
references on ignition systems recommend long duration times of 500 usee, or 
.joaore, where possible. However, an example of the ability of the capacitive 
portion of the discharge to initiate combustion has previously been demonstrated 
with the piezoelectric ignition system developed by Clevite Corporation in the 
early 1960's. The entire pulse width of the system was only 860 nsec. which 
is less than the rise time of a conventional magneto. 

Theoretical discussions on ignition systems indicated that a system 
with such short duration would not fire the mixture, but the piezoelectric 
system does fire the mixture and very well too. 

Due to combustion chamber variations from engine to engine, and 
even from cylinder to cylinder in the same engine, it is recommended that 
the spark duration be at least 100 to 200 usee, in duration. If the combustion 
chamber and engine design are satisfactory this duration will be sufficient to 
ignite the fuel-air mixture. 

4, Voltage Rise Time 

From examining various ignition systems, their past history and 
theoretical arc considerations, it appears that voltage and energy are not the 
only criteria of ignition system operation. Many of the systems that are capable 
of firing fouled spark plugs exhibit shorter rise times than the battery-coil 



20 



HP 




Specified 



12 gsec 



Ret. Adv. — ^ 

Ignition Timing 

Fig. 8 



20-- 



Energy 
(mJ) 




20 



60 
Capacitance ipF) 



100 



Fig. 9 



21 



system. It is concluded that rise time must be included in ignition system 
evaluation and design. For example, in referring to the piezoelectric system 
mentioned earlier, the rise time for this system was not measured exactly but 
was less than 10 nsec. Some investigations indicate it may be as short as one 
nsec. This short rise time system ran an engine six times longer between plug 
replacement than a longer rise time magneto; yet, in some circles, the magneto 
is thought to be the ultimate. 

Rise time is defined as the duration required for the voltage to 
build up and fire a spark plug. If this time is short, there is less opportunity 
for the energy to be dissipated in carbon deposits, moisture, and other partial 
conducting paths. Also, a short rise time is more effective in ion formation. 

Paschen's Law must again be evaluated in the realm in which it was 
written, namely that the electric field is uniform and that the voltage is slowly 
applied. For a short rise time pulse, the gap voltage may be reduced below 
the value considered normal by at least 15 percent. 

Shorter rise times have the disadvantages of larger radiation losses 
(increased radio interference), increased requirements on the system to pre- 
vent crossfiring, and more chance of developing unwanted carbon paths. 

An optimum ignition pulse would have sufficient rate of voltage rise 
to permit firing of heavily fouled plugs without the need for large total pulse 
energy. The rate of rise should be consistent with the voltage breakdown of 
the ignition system components. A rise time between 10 and 30 usee, should 
prove adequate for most cases. 



22 



5. Energy Requirements 

Total energy is important, in that a certain minimimi energy is 
required for ignition, but the required energy depends to an extent on the rise 
time and pulse width of the arc. Energy levels higher than necessary to account 
for the variables are detrimental to spark plug life. Since the energy level 
required for the standard mixture may be as low as 0. 002 mJ. , typical igni- 
tion system energies are higher than necessary. In general it is considered 
that 1 mJ. is sufficient to produce ignition of the fuel-air mixture. 

The energies mentioned above are those required to raise a small 
amount of mixture to combustion temperature. Further, this quantity of energy 
is actually a very small part of the total energy a system must have. Literature 
on system requirements list system energies from 10 to 40 mJ. These large 
energy requirements, compared to that required for combustion, are due to 

system losses and capacitances. If losses are neglected, energy requirements 

2 
reduce to E = ^CV . Assuming that under all operating conditions 22 kV. is 

sufficient to arc across the gap. Fig. 9 shows the energy required to overcome 

system capacitance. If a short rise time system is used little energy will be 

dissipated prior to the arc. Once the arc is struck, the capacitive energy is 

released rapidly in the leading section of the arc; inductive energy is released 

slowly increasing arc duration. 

Energy requirements should be held to a minimum. If excess energy 

is delivered to the arc, no improvement in combustion is noted, furthermore, gap 

erosion increases. Designers of ignition systems must consider the fact that 



23 



only a small, insignificant part of the total energy is required to produce 
combustion. The majority of system energy is used to assure that the required 
voltage will be developed. 



24 



in. IGNITION SYSTEMS 
The contents of this section describe the operation and characteristics 
of some of the ignition systems in use today in the automobile. Two ignition 
systems that are not in wide spread use are also discussed. This section 
covers both the non- electronic and electronic systems. 

A. KETTERING IGNITION SYSTEM 

Since 1914, automakers have used the Kettering or inductive ignition 
system — a battery, ignition coil, and cam-driven mechanical switch. Most 
of the automobiles sold in the U. S. come equipped with this system. Fig. 10 
is a representative schematic diagram of a typical Kettering system. 

Referring to Fig. 10, when the ignition switch SWl and the cam operated 
contacts SW2 close, current will flow through primary P of ignition coil T, 
building up magnetic flux. The current will reach a maximum value limited by 
the resistance of the primary. As the cam rotates, contacts SW2 are separated 
by the cam lobes, interrupting primary current. The distributor contact 
capacitor CI suppresses contact arcing and forms an oscillatory circuit with 
the equivalent primary inductance of T. 

Interruption of the primary current causes the flux in the ignition coil to 
collapse. The collapsing flux self-induces a voltage in the primary and by 
mutual coupling induces a voltage in the secondary of T. The secondary volt- 
tage is related directly to the primary voltage as the ratio of the nimiber of 
turns in the secondary to the number of turns in the primary. 



25 



sw 




Fig. 10 




Contacts 



Cs 



Vs 



Fig. II 



26 



The high voltage causes a spark to jump the small gap between the rotor 
R and the distributor cap insert with which the rotor is aligned, thereby firing 
the desired spark plug, SP, connected to the insert by a high tension cable. 

Figure 11 is also a schematic of the Kettering system, but contains 
some of the distributed components that are often overlooked. Cs is the 
capacitance of the secondary of T and the high tension leads. Rp and Rs repre- 
sent the internal primary and secondary resistances, respectively, of the 
induction coil and Cc is the contact capacitor. 

If a perfect ignition coil is assumed which is free from loss due to resist- 
ance, radiation, and dielectric hysteresis, the coil output energy will be equal 
to the coil input energy. The input energy in an ignition coil is related to the 
primary inductance and primary current by the equation: 
W =■! Lplp^ 



where: W = energy stored in primary 
Lp = primary inductance 
Ip = primary current. 

The current in an inductive circuit is a function of the time the circuit is 
energized. If the ignition, coil energy is not to decrease by more than 10 percent 
at some high speed value, the time constant, T = L/R, of the coil primary 
circuit must be one-third of the high speed primary circuit actuation time 
(contacts closed for three time constants). 

It is necessary to evaluate the total energy required in the primary of a 
coil. Assuming an ideal coil, to elevate the secondary to spark plug firing 



27 



voltage, 

Wt = ^ [Cs + (Np/Ns)^Cs] Vs^ (1) 

where: Wt = total energy in system 

Cs = total secondary capacitance 

Vs = secondary voltage 

Ns = turns on coil secondary 

Cc = contact condenser capacitance. 

From the above analysis a typical standard ignition system requires a 

primary current of- 16 amperes. This current must flow through the contacts, 

however, it is impractical for a contact set to handle this much current on a 

continuous basis. Therefore, current limitation forces conventional systems 

to operate at a maximum of about 5 amperes with a resulting decrease in high 

speed performance. 

12 
The disadvantages of the Kettering system are: 

1. The large value of current being interrupted by the contact- 
breaker points, cause excessive erosion. 

2. The moving arm of the contact-breaker tends to bounce at high 
speeds, thus shortening the time the points are closed. Point bounce reduces 
coil output and also increases point wear. 

3« A substantial reduction occurs in the output voltage with increas- 
ing engine speed, 

4. The system is highly inefficient at low engine speeds due to the 
high current. 

5. The system has a long voltage rise time resulting in poor 
performance when spark plugs become fouled. 



28 



B. HITACHI IGNITION SYSTEM"^ 

Examining Fig. 12, it is seen that this system is a modification of the 
Kettering ignition system described previously. The spark voltage is developed 
in an identical manner and energy requirements remain the same. The system 
also has the same disadvantages. 

By using the Hitachi ignition system, a 4-cylinder, 4-stroke engine can 
have the proper ignition sequence without the need for a distributor, and requires 
only the ignition coil and breaker-plate. The high tension cables are connected 
directly to the spark plug from the induction coil. By using the dual system as 
shown, dwell time is doubled over that for a Kettering system on a 4-cylinder 
engine. This improves high speed performance since primary current will 
have a longer time to build-up to the design value. 

The system has some major drawbacks that require discussion, dealing 
with the way in which the voltage is delivered to the plugs. Notice that two plugs 
are fired simultaneously in series with respect to the induction coil termination. 
This is permissable, since one plug fires on the power stroke while its mate 
fires on the exhaust stroke. 

The disadvantage is that higher potential must be developed to produce 
arcs in two plugs in series instead of just one. Since one plug is firing on the 
exhaust stroke, the potential required will be much less than that for the plug 
firing on the compression stroke. The main disadvantage, however, is that one 
plug is being fired with reverse polarity. As mentioned earlier, a plug fired with 
reverse polarity requires a few thousand volts more . 



29 



Contact- Breaker 



Cc 




<f 



d 



Cc 



Tl 



T2 



1 

I 

i 



1 

I 
i 



Fig. 12. Hitachi Ignition System 




► O- 



SP 



Fig. 13, Transistor Ignition 



30 



This system must still adhere to requirements that ignition voltage be 
between 25 kV. and 30 kV. To compensate for the need for higher ignition 
voltage, the spark plug gap is reduced slightly to lower the arc-over potential. 
This reduction in gap width will of course decrease the arc area available for 
the fuel-air mixture to circulate. 

The reason for considering this particular ignition system is that an 
electronic ignition system was designed in this study to replace it. 

C. PIEZOELECTRIC IGNITION 

The piezoelectric ignition system has not been commercially used on 
production engines. Its introduction here is to inculcate the point that rise 
time, being of little concern to ignition system designers until recently, should 
play a larger part in the design of systems and to point out that extremely long 
arc durations in the thousands of microseconds are not required to produce 
combustion of a homogenous fuel-air mixture. 

This ignition system derives its name from the piezoelectric generation 
of electricity in a crystal structure when pressure is applied. System operation 
is exceptionally simple in theory. The potential difference generated by a 
crystal, a stack of crystals in series, when struck sharply by mechanical 
means is applied to the spark plug. The voltage rise is extremely rapid and 
the energy delivered to the arc is strictly capacitive, therefore, the arc is of 
short duration. As mentioned before, the characteristics for this system are 
in the nanosecond range. 

Older theoretical discussions indicate that an ignition system with this 
short rise time and pulse width can not fire the mixture. This system has 

31 



run an engine six times longer between plug replacement than has a magneto with 
its long rise time and arc duration. It can start an engine after the spark plug 
has been soaked in water and put into the engine dripping wet. 

D. TRANSISTORIZED IGNITION SYSTEM 

The use of a transistor switch was one of the first attempts to use semi- 
conductors to improve the Kettering ignition system. The transistorized 
system is essentially identical to the conventional one except for the addition 
of transistor Ql, Fig. 13. The discussion of the Kettering system, section 
IIL A. , is applicable. 

The difference is that the primary current is now switched on and off by 
a transistor instead of the contact-breaker. The contact condenser is also 
eliminated with only the small collector to emitter capacitance in the primary 
circuit. The points control the base current thus turning Ql on and off. The 
small current in the base does not cause the points to erode as rapidly as in the 
Kettering system. A light or magnetic sensing device can be connected to the 
base of Ql to turn it on and off, thus eliminating the points entirely. 

By eliminating the points, a larger primary current can be used to 
improve high engine speed performance, the higher currents being obtained 
by reducing the primary inductance. This usually results in an increased 
turns ratio. Standard Kettering ignition systems usually have a turns ration 

of 100:1 while the transistorized systems have a much higher turns ratio, 

14 
often in the vicinity of 250:1 to 500:1. 



32 



Although the transistor ignition system produces a more constant voltage 
throughout the engine speed range, it still has a long rise time and puts a large 
demand on the battery and charging curcuit due to the requirement for increased 
primary current. Under starting conditions, this system may not perform^ 
satisfactorily. During cold weather operation it is often inferior to the battery- 
coil ignition system. 

The transistorized system was used on some production automobiles. It 
was soon discontinued since the small improvement in ignition high speed per- 
formance did not warrant its additional cost. Cold weather starting reliability 
was also less than the standard system, 

E. CAPACITOR DISCHARGE IGNITION SYSTEM (CDI) 

Capacitor discharge ignition systems have been on the market for a 
number of years. The CDI system was used as an electronic ignition long 
before transistorized ignitions were introduced. Figure 14 is a block diagram 
of a typical CDI system. In the early systems, the dc-to-dc converter was of 
the mechanical vibrator design and the gate was a thyratron tube. It was unre- 
liable, yet produced superior ignition. 

The CDI system remained obscure until the advent of semiconductor com- 
ponents. With the introduction of power transistors and the SCR, the dc-to-dc 
converter was easily produced using blocking transformers and the thyratron 
was replaced by its counterpart, the SCR, This ignition is challenging the 
Kettering system as the one for todays high performance engines, in fact, one 



33 



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34 



auto manufacture is using a GDI system as standard equipment on one of its 
1972 models. It had been offered as an option at a substantial price prior to 
this time. 

1. Operation 

There are two ways in which to use the ignition coil to produce the 
high voltage pulse. First is the rate-of- change of current, or inductive mode. 
This is the mode in which the Kettering system operates. Second is the trans- 
former mode. In this mode the coil acts only to transform a low voltage to the 
high voltage required. It is this second mode in which the capacitor discharge 
system functions. 

The dc-to-dc converter increased the low battery voltage to an inter- 
mediate level of a few hundred volts. The output charges the capacitor, referred 
to as a storage capacitor, to the intermediate voltage. At the proper time for 
ignition, the trigger circuit opens the gate which in turn connects the storage 
capacitor across the ignition coil. The capacitor voltage is then multiplied by 
the transformer's turns ratio to produce the high voltage for ignition. By using 
the transformer mode a much shorter rise time can be developed. The ignition 
coil can be designed to have low inductance and thus act as a pulse transformer. 
This can result in an ignition system with an extremely short rise time. 

Ignition coil primary pulse duration is shorter than the response 

time of the secondary. This means that even a capacitor storage type system 

15 
does store some energy in the transformer magnetic field momentarily. If it 

was not for this magnetic field storage, the capacitor energy would be delivered 



35 



very rapidly to the arc resulting in a very short duration pulse. It is the energy 
stored in the inductance that extends the arc duration since, as mentioned 
earlier, inductive energy is released slowly. 

Storage capacitor Cs must supply energy for the same reasons as dis- 
cussed previously in regard to inductive systems. 

Conduction of the gate connects Cs through the reflected impedance of 
the transformer to the secondary capacitance of the ignition system. Equating 
the energies on both sides of the perfect transformer and solving for Cs yields: 

Cs = Vs ^ Cd 

(2) 

Vp^- Vs^ (Np/Ns)^ 

where: Cs = energy storage capacitance 

Vs = secondary voltage 

Vp = Cs voltage before SCR conducts 

Cd = secondary distributed capacitance 

Np = primary turns, ignition coil 

Ns = secondary turns, ignition coil. 

- The above relationship holds for all values of voltage and capacitance when 

losses due to imperfect transformer action are neglected. 

2. CDI, Improved Ignition Characteristics 

The CDI system stores the energy required for ignition in a capacitor. 

This form of storage has the advantage that once sufficient energy has been 

accumulated in the capacitor, no more energy is consumed by the system until 

the capacitor has to be recharged for the next firing. This means that the 

system will draw only the energy it needs and therefore, current requirements 



36 



will vary as engine speed. In terms of energy requirement verses speed, the 
CDI system has a higher efficiency and improved utilization of energy drawn 
from the battery. 

The CDI system, with its shorter rise time, provides better perform- 
ance in firing fouled plugs. P. C. Kline of Delco-Remy reports that their 

experience with the Delco CD system shows plug life 4 to 5 times longer than 

9 
with conventional ignition. FIAT, on their sports cars, used the CDI in 

extending the thermal range of the spark plug so that a cold spark plug can be 

used for highway operation, and at the same time avoid misfirings due to cold 

9 
fouling at low speeds. 

One definite benefit is improved starting, particularly in damp 

weather, or in very cold weather. CD equipped engines tolerate carburetor 

flooding and other problems that cause starting difficulties. 

Champion Spark Plug Company studied the effect of capacitor discharge 

ignition on electrode erosion. Champion noted that spark plug gap growth was 

much less when using the capacitor discharge ignition system with the fast rise 

time and short arc duration. In fact, the spark plug gaps from the capacitor 

discharge system actually decreased slightly due to a light deposit build-up. 

Gap growth measurements are not a true indication of overall deterioration. 

Center electrodes from the conventional system were round while the electrodes 

from the capacitor discharge system still had relatively sharp edges. Sharp 

edges are desirable since they reduce the voltage required for arc production. 

The trigger circuit for a CDI system can be designed using a light or 

magnetic sensor instead of the customary contact-breaker. 



37 



An ignition system having the above characteristics, if its electronics 
were properly designed, would require a great deal less maintenance than the 
standard battery-coil ignition. Since the contact-breaker assembly could be 
eliminated, the only wear would be in the shaft bearings of the distributor 
assembly. Once ignition timing was initially set, it would not need resetting 
unless major maintenance was necessary on the distributor. The need to re- 
move spark plugs for cleaning, regapping, or replacement is greatly reduced, 
thus, greatly extending plug life. 

The CDI system has the characteristics that are badly needed on todays 
automotive engines. This system, if properly designed, could be the long-needed 
replacement for the 1914 Kettering ignition system. 



38 



IV. IGNITION SYSTEM DESIGN REQUIREMENTS 

The contents of this section define the requirements one must consider in 
the design of ignition systems. Presented are current ideas on ignition system 
design. Applying these concepts will result in systems which appear to be 
better solutions. However, some procedures, if followed, mi^t give only a 
brief reprieve from the problem the system was to eliminate before introducing 
problems of its own. 

A. BRUTE FORCE CRITERIA 

If some is good, more must be better ! R. G. Van Houten and J. C. 
Schweitzer of Delta Products states, "Any new ignition system must meet the 
following requirements:" 

1. "Output energy levels should exceed present levels by substantial 
margins. A new system should be able to develop energies of 40 milliwatt- 
seconds minimum, and be easily controlled to set this level higher if necessary. 
The energy output and voltage levels should remain constant, over an rpm range 
of 8, 000 to 10, 000 on eight- cylinder engines. " 

2. "As rapid a voltage rise time as possible." 

17 

3. "It should be low cost and designed for high volume production. " 

In the description of another system, the following statement is made, "It 
has been pretty well established that a minimum of 30 milliwatt- seconds of 

energy is required at the spark plug in modern ignition systems. CI has been 

18 

chosen to give 80 milliwatt- seconds, allowing ample reserve energy." 



39 



Again the system designer thinks in terms of brute force, "... we find 
that it takes about 40 kV. to operate the spark plugs. This 40 kV. should be 

considered a minimum requirement. To assure complete combustion, this 

14 
value should be exceeded if possible. " (Manufacturers of electronic ignition 

systems have advertised voltages as high as 60 kV. ) 

Referring to previous discussions on ignition system requirements, energy 
requirements are related more to system losses than to the energy required to 
ignite the fuel-air mixture. Energy requirements necessitate careful system 
evaluation and not the setting of a blanket value. The energy is not held in 
reserve as mentioned above, but any excess energy, over that required to 
ignite the mixture and compensate for system losses, is dissipated in the arc 
and leads to excessive electrode erosion. Some German aircraft during World 
War II used a high energy CD system to facilitate cold starts. Because of the 
high energies involved, spark plug life was only 25 hours. 

Under normal operation, a spark plug requires only about 4 to 8 kV. to 
produce an arc. However, since the engine will be operating under various 
load requirements, a voltage of 22 kV. is considered ample. 

B. GENERAL DESIGN REQUIREMENTS 

Following is a number of design criteria to be considered in the design 
of ignition systems. 



40 



1. Use as short a voltage rise time as practical, not necessarily as 
short as possible. With a sufficiently short rise time, an ignition system can 
more readily fire fouled plugs. In selecting the upper limit on rise time, 
capacitance loading, corona loss, and insulation failure become of paramount 
importance. 

2. A new ignition system must be more reliable than the system it 
replaces. Reliability includes the time and cost of maintenance. 

3. If the system is not original equipment, installation should require 
a minimal change in components or wiring. 

4. Input power should vary as engine speed. 

5. Use energy levels only sufficient for operation. 

6. Gains should be made by well known ignition practices related to 
voltage, namely: 

a. Keep the capacitance of the ignition leads as low as possible 
by keeping them away from metal parts. 

b. Reduce secondary series resistance to that required for radio 
suppression. 

c. Use short leads. 

d. Reduce corona losses, and hysteresis of coils and capacitors. 

e. Develop only sufficient voltage to assure that an arc can be 
produced at all engine load and operating conditions. A small voltage reserve 
may be applied, but should not be overdone. 

7. If the system is designed to replace an existing one, leave the 
original system intact so that it may be readily reconnected in case the new 

system fails. 

41 



8. Be able to operate at temperatures in the engine compartment, 
preferably as high as 250° F. , to permit installation directly on the fire wall, 

C. SPECIFIC REQUIREMENTS FOR A SYSTEM WHICH IS TO REPLACE THE 
HITACHI IGNITION SYSTEM 

Figure 27 is a table of the characteristics of the Hitachi system. The 
design parameters for the CDI replacement system conform to, or are im- 
provements on, the Hitachi parameters. 

1. Maximum Voltage Requirements 

The ignition system was designed to supply 25 kV. optimum but less 
than 30 kV. to protect high tension components. 

2. Spark Plug Gap 

The CDI, due to shorter arc duration, uses a wider gap than the con- 
ventional system. The Hitachi system gap is set at 0. 6 to 0. 7 mm. — for the 
CDI system the gap was widened to 1. mm. 

3. Spark Duration 

Arc duration was selected as 200 usee, to assure consistent ignition. 
The storage capacitor was varied until a value was established that optimized 
between spark duration and energy required. 

4. Storage Capacitor and Energy Requirements 

By equating energy on both sides of the ignition coil, equation (2) 
was derived. This was used in calculating the energy storage capacitor, 
which in turn was used to establish how much energy was stored. The param- 
eters used were: 



42 



Cs = 0. 2 uF. from equation (2) 
Cd = 20 pF, 
Vp = 600 V. 
Np = 380 turns 
Ns = 15,000 turns. 

The stored energy calculated, 36 mJ. , is close to the 30 mJ. standard 

discussed earlier. 

5, Rise Time 

Rise time of the voltage pulse applied to the high tension circuit will 
not be shorter than 10 usee, so that corona and radiation loss will be held to a 
minimum. Rise time is to be no longer than 30 usee, to reduce energy loss and 
high speed timing error. 

There is no direct way to control rise time in the design of this 
system since the original ignition coil is used. By the use of a capacitive dis- 
charge through the coil, response of the coil was improved resulting in a 
shorter rise time. 

The use of the original ignition coil, rather than a specially designed 
transformer, was one of the factors evaluated. 

6. Power Input Requirements 

In section IV. B. 4. , a storage energy of 36 mJ. per ignition pulse was 
calculated. Maximum input power is required at maximum engine speed. The 
ignition system must fire a 4-cylinder, 4- stroke engine, delivering peak bhp. 
at 8500 rpm. A design margin of 1500 rpm. is included yielding a maximum 
design rpm. of 10, 000. 

At maximum rpm. the ignition system requires, for storage capac- 
itor energy, 12 watts assuming 100 percent efficiency. Taking into consideration 



43 



the efficiency of the dc-to-dc converter, the power to operate the trigger 
circuit, and sufficient current to keep the contact-breaker clean, an efficiency 
of 70 percent is assumed. Thus, system power input is less than 17 watts at 
an engine speed of 10, 000 rpm, 

7. Summary of Design Criteria 

Listed below is a summary of the requirements considered as goals in 
the design of the Hitachi replacement: 

1. Maximum engine speed is 10,000 rpm. 

2. Maximum high tension voltage between 25 to 30 kV. 

3. Arc ionization duration is 200 usee. 

4. Rise time, 10 to 30 usee. 

5. Capacitor storage energy, 30 to 40 mJ. 

6. 15 to 20 watts of power consumption from a 12 V. dc. system, 

7. Use the original ignition coil, 

8. Operate over a temperature range of to 80*^ C. 

9. Design for limited area installation. 



44 



I 



V. FEASIBILITY STUDY 

Before an attempt was made to design a CDI system, it was realized that 
a pre- evaluation should be considered to test the feasibility of designing the 
circuit around the parameters listed earlier. To accomplish this task, the 
IBM 360/67 digital computer was used in association with the IBM circuit 
analysis program LISA. During the computer evaluation, circuit parameters 
were varied to determine their effect on system performance. 

To have a means of comparison, the Hitachi ignition system was evaluated, 
followed by the CDI evaluation. 

A. HITACHI IGNITION EVALUATION 

The Hitachi system was modeled as shown in Fig. 15. Initial condition 
primary current was calculated from the time the ignition points are closed. 
The computer solution commenced at the time of point opening. 

LISA uses linear nodal analysis techniques. Since the characteristics of 
an arc are highly nonlinear, a complete solution was unobtainable. The spark 
plug was replaced with a load resistor, then the computer gave an indication 
of the system rise time and available ignition voltage. Fig. 16 is a table con- 
taining data from the solution. 

B. CDI SIMULATION 

Of interest was the question, could a capacitor discharging through the 
primary of the Hitachi ignition coil produce the desired results listed earlier? 



45 



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To answer the question, the GDI system ^\as modeled as shown in Fig, 17. Rg 
and impulse driver Vg were used only to implement LISA. Again, due to the 
nonlinear properties of the arc, the spark plug was replaced with a resistance. 

In the analysis it was assumed that the initial condition voltage of the 
storage capacitor was not affected by engine rpm; this is valid as long as the 
charging source has a sufficiently low impedance. Therefore, solutions were 
not obtained for varying engine speeds. However, the storage capacitor value 
was changed along with other circuit components to evaluate their effects on 
ignition output. 

The computer output verified the selection of a 0. 2 uF. storage capacitor. 
A portion of the solution is shown in Fig. 18. Referring to Fig. 18, the rise 
time falls between the limit set but the output available voltage is slightly 
higher than the 30 kV. upper limit. 
C. CONCLUSION 

The computer analysis demonstrated the superior operation of the CDI 
systen o\er the battery -coil system even when the same coil was used for both 
applications. The solution indicates that a system could be designed to meet 
stated specifications. 

On the basis of the computer output, it was decided to proceed and to 
design a CDI system to replace the Hitachi. 



48 



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50 



I 



VI. CAPACITOR DISCHARGE IGNITION SYSTEM DESIGN 

System design was based on the requirements presented in section IV. B. 7. 
which were adhered to except when situaticns developed that required modifica- 
tions. 

Reviewing Fig. 12, one sees the Hitachi ignition system consists basically 
of two Kettering systems each producing an ignition pulse 180 degrees of engine 
rotation apart. The system can be considered as two independent systems con- 
nected together, for ignition timing, by a common breaker cam. Therefore, 
two independent, identical, CDI systems could be designed to replace the 
battery-coil system. 

To produce a more compact, efficient, and lower cost system, it was 
decided to design a system having one converter and energy storage capacitor. 
Here, the circuit branched into two parts, each having its own ignition coil, 
gate, and trigger circuits. The system block diagram is shown in Fig. 19. 

A. DC-to-DC CONVERTER 

The function of the dc-to-dc converter is to raise the low battery voltage 
to an intermediate value to charge the energy storage capacitor. 

Basically, the converter circuit will require more space and determine the 
over-all efficiency of the circuit. To reduce its size and increase its efficiency, 
the non- saturating circuit was selected over the saturating type. Generally, 
however, the non- saturating circuit is more complex since it requires an ac 
power source to excite the transformer. Fig. 20. 



51 



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52 



To further reduce the size, a relatively high frequency of 10 kHz. was 
selected, which permitted the use of a small powdered iron core transformer. 

Originally, an intermediate voltage of 600 volts was selected. However, 
an off-the-shelf inverter transformer was available that transformed the 12 V. 
battery voltage to 560 V. Referring back to the computer solution, the output 
voltage exceeded the 30 kV. limit, thus, the lower intermediate voltage will 
lower the output voltage. 

Total time between firings is 3 msec, at maximum engine speed. To 
assure that arc ionization has ceased and transformer ringing decreased to 
where the gate can revert to an off state, half of the 3 msec, was allotted for 
the above. This leaves 1.5 msec, charging time. Since maximum bhp. is 
developed at 8500 rpm. , the storage capacitor voltage was allowed to degrade 
to 450 V. at 10,000 rpm. 

Referring to Fig. 20, transistors Ql and Q2 act as constant current 
sources until C3 has charged to about the design value. This is due to the 
capacitive load on the converter transformer reflecting into the primary as a 
very low impedance. Assuming a constant current charge, C3, will charge to 
500 V. in 1.5 msec, if the secondary of Tl supplies 66.7 mA; this requires 
a primary current of 3 A. 

The astable multivibrator drives Ql and Q2 at 10 kHz. Ql and Q2 drives 
Tl with 3 A. during the charging of C3. When C3 has charged to design value, 
the output transistors saturate and primary current drops to the minimum 
value sufficient to keep C3 charged. 



53 




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As mentioned earlier, the dc-to-dc converter is gated on and off. This is 
accomplished by diodes D3 and D4, When terminal G is high, approximately 
Vb, the diodes are in the off state, and the MV functions normally. If terminal 
G is low, approximately ground potential, D3 and D4 conduct and clamp Q3 and 
Q4 bases at ground potential biasing Ql and Q2 off. 

Due to the low resistances in the MV circuit it has a tendency to block. 
The addition of diodes Dl and D2 prevent blocking. 

B. DISCHARGE CIRCUIT AND MV GATE 

The schematic diagram of the discharge circuit and MV gate is shown in 
Fig. 21. When a positive pulse is applied to the gate of the SCR it conducts 
causing C3 to discharge through the primary of the ignition coil. Before the 
SCR will revert to the off state, current must be less than the holding current. 
Q5 is used to apply a turn off pulse to the converter during the time the SCR 
is conducting. If the converter is not shut down during the discharge portion 
of the cycle, it v^ll supply sufficient current to hold the SCR on continuously 
until complete system shut-down. 

The MV gate is able to completely turn off the converter over the range 
of minimum holding current, 0. 5 mA. , to peak primary current of 3. 52 
amperes. The gate is basically an RTL circuit with a NOR positive logic 
function. Either input, when either SCR conducts, clamps the gate output to 
ground potential, thus, biasing the MV off. 

Since the SCR current varies over a large range, the current sensing 
device should be nonlinear producing a sufficient signal at low current to 



55 



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56 



keep Q5 saturated, yet at high current the sensor output will not damage the 
base-emitter junction of Q5. This requires a device having high resistance at 
the holding current but low resistance at the peak primary current. 

To accomplish this, diodes D9 through D12 were connected as shown. At 
low currents, the diodes will not conduct unless cut-in voltage is exceeded. 
The cut-in voltage for the two diodes in series is sufficient to hold Q5 at satu- 
ration. The voltage drop across the diodes when peak current is applied remains 
sufficiently close to the cut-in voltage to prevent excessive base current in Q5. 

C. SCR TRIGGER CIRCUIT 

The original breaker-plate for the Hitachi system remains as the ignition 
timing source. This amplifies mechanical alterations and permits the ignition 
to be readily switched back to the original battery-coil system if the CD I system 
fails. 

The unijunction transistor, UJT, trigger circuit is shown in Fig. 22. The 
circuit produces a 5 V. peak pulse of 50 usee, duration. Referring to Fig. 23, 
it is seen that the output of the trigger circuit is capable of triggering the SCR 

over a wide temperature range. The SCR operates within its dissipation limits 

19 
and, Fig. 24, has a turn on time of 1 to 3 usee. 

UJT Q6 will not conduct as long as the ignition contacts are closed. When 

the contacts open, Q6 conducts and charges C4 through R12, Q6, D14, and the 

low resistance of the SCR gate. After approximately 5 time constants, the pulse 

applied to the SCR has returned to zero. Therefore, the values of the above 

components determine the RC time constant, thus, the pulse duration. When 



57 



the points close, C4 and R14 are chosen so that the UJT can not be retriggered 
r until 1 msec, has elapsed, thus preventing false triggering during the time the 

points have a tendency to bounce. 

Resistor R12 is selected to provide sufficient current flow to keep the 
points clean, but, not to cause excessive erosion. 

A complete schematic diagram of the GDI system is shown in Fig. 25. 



58 



RIO 



T 

Vb 



RIA 



C4 



"W 



Q6 



DI4 



^Tl 




RI2 



RI6 



RI8 



Rll 



C5 



T 

Vb 



RI3 



RI5 



^^v^l ' D15 

^ Q7 , 




RI7 



■^12 



RI9 



C4,C5 = 0.1 pf 
DI4,DI5=1N270 
Q6,07- 2NI67I 
RIO, Rll= IIOXL , 2W 



RI2, RI3= II K 
RI4,RI5= 500iL 
RI6, RI7= 220iL 
RI8, RI9= I.IK 



Fig. 22. U JT Trigger. 



59 



FORWARD GATE CHARACTERISTICS 



!00 

8 

6 
4 



SHflOEP JA INDICATES LOCUS 

OF PC, ;.6LE TRIGGER'NG POINTS 
_FCR V--..,;uS rCWPEHAru-»ES 



> 
I 6 



o 2 
O 

X 



-H-PrH 



I ; I i 

-: I ' . 



irrr 



—I — :- 

^'21.''"^' * 'PERMITTED PULSE WIOTHS 
, , y----T— FOR INDICATED "^EAK 

-^ — '-t-f — - <■■ FORWARD GATE POWER 



-MAXMUV GATE TRIGGER 
- VOLTAGE f^C If.DICATED 
^JUNCTION TE\'PERArur,E ( 
T,=-40*C -^ r 




MAXIMUM VULPAGE AT WHICH 



MAXIMUM VULFAGE AT WHICH. ^' i ■ • , , , 

■NO UNIT W.ui. 7f.:;GGch FOR I- ,- ,1 i 4 | -Ul-L.-I 
Tj-^-HOO»C ' : 'I ll i I 



0.001 



GATE -TO -CATHODE CURRENT — AMPERES 



Fie. 23. 



TURN-ON TIME CHARACTERISTICS 



^[|| ! i | !!l!|! i : 4 ::ritn -rH^t^H-^ - l ' n : : n i ' ! 



o 
* 2 

i 

I 



z I 
o 




^ rfPt ' i ! I ; i I 



03 0.5 0.7 

GATE CURRENT (Igt)-AMPERES 



0.9 



92CS-I3S59«1 



Fig. 2k 



60 




a 
u 



in 

CM 



61 



VII. GDI SYSTEM TEST AND EVALUATION 

The output of the breadboarded GDI system was measured and its charac- 
teristics compared to the desired specifications listed earlier and the Hitachi 
output. 

The bench set-up consisted of a small battery identical to the one used for 
the Hitachi ignition and a breaker plate assembly mounted on a universal motor 
whose speed was variable from 1,000 to 10,000 rpm. The GDI system was 
connected to spark plugs mounted on a manifold that was pressurized with nitro- 
gen. Fouling was simulated by placing 500 kxi non-inductive resistors across 
each plug. 

Nitrogen was chosen as the pressurizing agent since the earth' s atmosphere 
Is about four-fifths nitrogen and a source of nitrogen was readily available. The 
use of nitrogen was not selected to simulate the conditions that exist in the com- 
bustion chamber during engine operation but to give a controlled condition on 
which to compare the Hitachi ignition with the GDI system. 

By pressurizing the spark plug manifold with nitrogen worst case conditions 
were simulated, therefore, cold plugs in a "very lean" mixture. 

The following tests were performed on both ignition systems at 1, 4, and 
10 thousand rpm: 

1. Open circuit 

a. Amplitude 

b. Rise time 

2. 500 ki^ load 

a. Amplitude 

b. Rise time 

62 



3. Spark plug impressurized 

a. Amplitude of spike and table 

b. Rise time 

c. Table duration 

4. Spark plug pressurized to 15 psi. 

a. Amplitude of spke and table 

b. Rise time 

c. Table duration 

5. Spark plug pressurized and 500 ka parallel load 
. a. Amplitude of ^ite and table 

b. Rise time 

c. Table duration. 

Figure 26 is a record of the Hitachi ignition output measurements as related 
to the outlined tests. Figures 27 A through E are photographs of the output at 
an engine speed of 4, 000 rpm. On examining the figures it is seen that the 
ignition system failed to produce an arc for a simulated fouled plug at a high 
engine rpm. If the manifold pressure was increased to 30 to 45 psi. it pro- 
duced intermittent firing for test 4 and would not for any test produce an arc 
at high rpm. These bench tests indicated poor ignition performance under 
fouled plug conditions. 

The same tests were performed with the GDI system. Results are recorded 
in Fig. 28 and output photographs shown in Fig. 29 A through E. 

Key system waveforms are shown in Fig. 30 A through G and depict circuit 
operation. Figure 31 is a curve of power consumption as related to engine 
speed. One sees that power input increases with rpm, as desired and is within 
the value specified in section IV. C. 6. 



63 



The GDI system was capable of producing an arc when the plugs were 
pressurized to 125 psi, where the Hitachi system failed at pressures over 
40 psi. Thus the CD! system displayed superior performance over the 
Hitachi system when bench tested under identical conditions. 



64 



TEST 


- 


RPM 




CONDITION 


1,000 


4,000 


10,000 


Input Current (Amps) 


2.67 


2.3 


1.7 


Stored Energy (mj) 


53 


40 


22 


TEST I 








Voltage Amplitude (kV) 


24 


24 


16 


Rise Time (usee) 


44 


44 


44 


TEST II 








Voltage Amplitude 


12 


10 


8 


Rise Time 


44 


44 


44 


TEST III 








Spike Amplitude (kV) 


4.4 


4.4 


4.4 


Table Amplitude (V) 


500 


500 


500 


Table Duration (usee) 


1300 


1300 


800 


Rise Time (usee) 


30 


30 


30 


TEST IV 








Spike Amplitude 


7.2 


7.2 


7.2 


Table Amplitude 


1000 


1000 


1000 


Table Duration 


1000 


1000 


1000 


Rise Time 


35 


35 


35 


TEST V 








^ike Amplitude 


7.2 


7 


5 


Table Amplitude 


500 


Inter- 


None 


Table Duration 


800 


mittent 




Rise Time 


40 







Fig. 26. Hitachi Output. 



65 



VERT = lOkV/div 



ov- 



HOR = 0. 5msec/div 




Fig. 27A. Test I. Open circuit voltage, Hitachi System 



VERT = 5kV/div 



OV- 



HOR = 0. 2msec/div 




Fig. 27B. Test 11. 500 k load, Hitachi System. 



66 



VERT = IkV/div 



OV- 



HOR = 0« 5msec/ div 




Fig. 27 C. Test III. Unpressurlzed Spark Plug Load. 



VERT = 2kV/div 



OV- 



HOR = 0. Smsec/div 




Fig. 27D. Test IV, Pressurized Spark Plug Load, 

Hitachi System, 



67 



VERT = 2kV/div 



OV- 



HOR = 0. Imsec/div 




Fig. 27E. Test IV. Simulated Fouled Plug, 

Hitachi System. 



68 



TEST 




RPM 




CONDITION 


1,000 


4,000 


10,000 


Input Voltage (V) 


560 


560 


425 


Stored Energy (mJ) 


31.4 


31.4 


18 


TEST I 








Voltage Amplitude (kV) 


28 


28 


28 


Rise Time (usee) 


20 


20 


20 


TEST II 








Voltage Amplitude 


12 


12 


10 


Rise Time 


20 


20 


19 


TEST III 




• 




Spike Amplitude (kV) 


5.4 


5.4 


5.4 


Table Amplitude (V) 


350 


350 


350 


Table Duration (usee) 


250 


250 


250 


Rise Time (usee) 


10 


10 


10 


TEST IV 








Spike Amplitude 


7 


7 


7 


Table Amplitude 


400 


400 


400 


Table Duration 


220 


220 


220 


Rise Time 


10 


10 


10 


TEST V 








Spike Amplitude 


7 


7 


8 


Table Amplitude 


500 


500 


500 


Table Duration 


220 


220 


220 


Rise Time 


12 


12 


15 



Fig. 28. GDI Output. 



69 



VERT = 20kV/div 



OV 



HOR = 0. 2msec/div 




Fig. 29A. Test I. Open Circuit Voltage, GDI System. 



VERT = 5kV/div 



OV- 



HOR = 0. Imsec/div 




Fig. 29B. Test II. 500 k Load, GDI System. 



70 



VERT = IkV/div 



OV- 



HOR ^ 0. 2msec/div 




Fig. 29C. Test III. Spark Plug Load, GDI System, 



VERT = IkV/div 



OV- 



HOR = Oo 5msec/div 




Fig. 29D. Test IV. Pressurized Spark Plug, GDI System. 



71 



VERT: 20V/div 
Collector: Ql 

VERT: IV/div 
Base: Ql 

HOR: 20usec/div 



OV- 



OV- 




Fig. 30A. Converter Output Transistor Wave Form. 



VERT: lOV/div 



Collector: Q3 



Base: Q3 
VERT: 5V/div 

HOR: 20usec/div 



OV- 



OV- 




Fig. SOB. Converter Driver Transistor Wave Form. 



72 



VERT: 5V/div 
SCR Gate 



Points 

VERT: lOV/div 
HOR: 2msec/div 



OV- 



OV- 




Fig. 30C. Trigger Circuit Wave Form. 



VERT: 200V/div 



C3 Voltage 



Voltage Spark Plug 



HOR: 2msec/div 



OV- 



ov- 




Fig. SOD. Storage Capacitor Chaigicg Wave Form, 



73 



VERT: 20V/div 
Collector Ql 



CDI Output 

VERT: IkV/dlv 
HOR: 2msec/div 



OV- 



OV- 




Fig. 30E. Gated Converter Drive to Inverter Transformer Wave Form. 



VERT: 2V/div 
SCR Cathode 



CDI Output 



VERT: IkV/div 



HOR: 2msec/div 



OV- 



OV- 




Fig. 30 F, MV Gate Input Wave Form. 



74 



VERT: 5V/div 



Cathode D3 and D4 



GDI Output 



VERT: IkV/div 



HOR: 2msec/div 



OV- 



OV- 




Fig. 30G. MV Gate Output Wave Form. 



75 




«> 





o. 




E 




3 


*-«. 


(A 


E 


C 


Ql 


o 


k. 


o 


O 






w. 


o 


<o 


o 


5 


_s 


o 


— ' 


a. 


•o 


E 


a> 


a> 


0) 


(/) 


o. 


>» 


to 


CO 


Q) 




c 


Q 


o> 


O 


c: 




UJ 


. 








ro 




O) 



<M 



(D 



CM 



o « 

Q. ^ 
o 

; E 



76 



Vm. CONCLUSIONS AND RECOMMENDATIONS 

An ignition system is required to replace the aging battery-coil ignition 
now used on practically every automobile engine in use today. The Kettering 
system is no longer able to meet modern automotive engine demands, and 
capacitor discharge ignition systems exhibit superior performance in producing 
ignition in today's high output engines. 

Energy and ionization duration requirements associated with the battery 
coil system cannot be applied to the CD ignition. New standards need to be 
derived to best utilize the advantages of the CDI system. Brute force engineering 
must be eliminated. A CDI system design must meet the conditions imposed by 
the engine on which it is to be installed in order to prevent an over or under 
design. Many CDI system design specifications far exceed the requirements 
necessary to produce ignition. 

The initial cost of a CDI system is considerably higher than that of the 
Kettering. However, maintenance expenses and inconveniences imposed upon 
the customer must also be considered. 

The trend in industry is to cut the production cost of ignition systems, 
which too often leads to systems having low reliability. To improve reliability, 
initial system cost should be allowed to rise; reduced maintenance will offset 
higher prices. The repayment is customer convenience, since CDI systems 
can be produced having one fourth the maintenance requirements of the old 
system. 



77 



Automotive history clearly refutes the belief that convenience features . 
cannot compete with cost. Certainly, no one can claim that the electric starter 
replaced the hand crank on a cost basis. The automatic transmission outsells 
the standard transmission because it is more convenient, not less expensive. 
High output eight cylinder engines are sold at a price premium in todays market 
on the basis of performance, obviously not on a lower cost basis. When the 
added performance and convenience can justify the added cost, the CDI system 
will be accepted. 



78 



LIST OF REFERENCES 

1. AC Spark Plug Service Manual , AC Spark Plug Division, General Motors 
Corporation, p. 14-18, Oct. 1968. 

2. Spaulding, G. E. Jr., Transistor Switched Igniton Systems , SAE Annual 
Meeting, p. 11, Jan. 11-15, 1960. 

3. Letter from R. E. Massoll, Spark Plug Engineering, AC Spark Plug 
Division, General Motors, Subject: Ignition System Requirements, 
22 Apr. 1971. 

4. Champion Ignition and Engine Performance Conference , Detroit, Michigan, 
1970, Automotive Technical Services Department, Champion Spark Plug 
Company. 

5. Champion Ignition and Engine Performance Conference , Sections I throu^ 
V, Toronto, Ontario, Canada, 1968, Automotive Technical Services 
Department, Champion Spark Plug Company. 

6. Stevens, Carlile R. , "Energy Storage and the Criteria for Proper Ignition 
in the Internal Combustion Engine," IEEE Transactions on Industrial 
Electronics and Control Instriunentation , vol. lECI no. 1, pg. 8-13, 
Mar. 1965. 

7. Engineering Manual, Champion Spark Plug Company, p. 22-23, 1966. 

8. Larew, Walter B. , Igniton Systems , p. 118-201, Chilton Book Company, 

9. Champion Ignition and Engine Performance Conference, Tokyo, Japan, 
1969, Automotive Technical Services Department, Champion Spark Plug 
Company. 

10. Eason, William R. , Voltage Risetime — A New Ignition Criterion , Society 
of Automotive Engineers, Automotive Engineering Congress, Detroit, 
Mich. , Jan. 14-18, 1963. 

11. Lovrenich, R. T. , "Automotive Ignition Problems Which Semiconductors 
Can Solve," IEEE Transactions On Industrial Electronics and Control 
Instrumentation, vol. lECI no. 2, p. 6-13, Sep. 1964. 



79 



12. Alexander, W. , and Sample, P. , "An Experimental Electronic Ignition 
System," Electronic Engineering , vol. 36 no. 442, p. 813-816, Dec. 1964. 

13. Shop Manual Honda CB 750 , Honda Motor Co. , Ltd. , p. 76-84, 1970. 

14. Carroll, Ronald L. , "Electronic Ignition Systems," Electronics World, 
vol. 77, p. 47-49, Feb. 1967." 

15. Warner, R. S. , Welch, H. C. Pelligrino, P., Brown, R.J. , and O'Neill, 
J. A. , "Solid State Ignition," Mechanical Engineering, vol. 88 na 10, p. 
30-37, Oct. 1966, 

16. Champion Ignition and Engine Performance Conference, Brussels, 
Belgium, 1967, Automotive Technical Services Department, Champion 
Spark Plug Company, 

17. Van Houten, R. , and Schweitzer, J. C. , "A New Ignition System for Cars," 
Electronics , vol. 37 no. 26, p. 68-72, Dec. 5, 1964. 

18. Ward, B. , "SCR Automotive Ignition System," Electronics World , vol. 72, 
p. 44-45, Nov. 1964. 

19. Specification Sheet on RCA 2N4103 SCR, file no. 116, Radio Corporation 
of America, May 1966. 



80 



INITIAL DISTRIBUTION LIST 

No, copies 

1. Defense Documentation Center 2 
Cameron Station 

Alexandria, Virginia 22314 

2. Library, Code 0212 2 
Naval Postgraduate School 

Monterey, California 93940 

3. Assist. Professor Richard W. Adler, Code 52Ab 2 
Department of Electrical Engineering 

Naval Postgraduate School 
Monterey, California 93940 

4. LT Terrence L. Williamson, USNR 2 
513 Coates Avenue 

Layton, Utah 84041 

5. Mr. Arthur Hoffman 1 
Magnetic Circuit Elements 

Henderson Way 

Monterey, California 93940 

6. Mr. Co Robert Smith 1 
Champion Spark Plug Company 

P. O. Box 910 
Toledo, Ohio 43601 

7. Mr. Richard E. Massoll 1 
AC Spark Plug Division 

GM Corp. Dept. 32-01 
1300N. Dort Highway 
Flint, Michigan 48556 

8. American Honda Motor Co. , Inc. 1 
100 W. Alondra 

Gardena, California 90247 



81 



9. International Four Owners Association 
1380 Garnet Avenue 

San Diego, California 92109 

10. Assoc. Professor Milton L. Wilcox 
Code 52Wx 

Department of Electrical Engineering 
Naval Postgraduate School 
Monterey, California 93940 

11. Prof. P. F. Pucci 
Code 59Pc 

Department of Mechanical Engineering 
Naval Postgraduate School 
Monterey, California 93940 



82 



Unclassified 



S<>L(iritv Cld'isifiration 



DOCUMENT CONTROL DATA -R&D 



\Securitv classificalion ol lille, hodv ol ab^tracl and indexing ennolfilion niu.sf be entered when the overall report /s classified) 


1 ORIGINATING ACTIVITY ( Corpora le author ) 

Naval Postgraduate School 
Monterey, California 93940 


2«. REPORT SECURITY CLASSIFICATION 

Unclassified 


2b. GROUP 



3 REPOR T TITLE 

IGNITION SYSTEM REQUIREMENTS AND THEIR APPLICATION 

TO THE 
DESIGN OF CAPACITOR DISCHARGE IGNITION SYSTEMS 

4 DESCRIPTIVE NOTES (Type o/ report and, inclusive dates) 

Master's Thesis; December 1971 

5 auThOR(S) (First name, middle initial, Imal rtame) 

Terrence Lyle Williamson 
Lieutenant, United States Naval Reserve 



« REPOR T D A TE 

December 1971 



7«. TOTAL NO. OF PAGES 



84 



76. NO. OF RE FS 

19 



•a. CONTRACT OR GRANT NO. 



6. PROJEC T NO. 



9a. ORIGINATOR'S REPORT NUMBER(S) 



96. OTHER REPORT NO(S) (Any other nuatbera that may be aaalgnad 
this report) 



10 DISTRIBUTION STATEMENT 



Approved for public release; distribution unlimited. 



II. SUPPLEMENTARY NOTES 



12. SPONSORING MILITARY ACTIVITY 

Naval Postgraduate School 
Monterey, California 93940 



13. ABSTR AC T 



Kettering ignition systems used on the majority of automotive engines can no longer 
assure reliable ignition for high-output engines. The capacitor discharge ignition, CDI, 
is a promising system to supersede the obsolete battery-coil. 

This study examines wave-front requirements at the spark plug for producing 
ignition in the internal combustion engine and system characteristics necessary for 
producing the wave-front. Arc requirements are described and used to define CDI 
parameters. 

A modified Kettering system which violates some basic ignition concepts was 
replaced by a CDI system designed in this study. Its characteristics were derived 
from arc requirements, not by aggrandizing the replaced battery-coil parameters. 

During performance tests, the CDI system exhibited superior performance. It 
fired simulated fouled plugs and continued to produce an arc when pressurized to 3 
times the value at which the Kettering ceased to function. This improved performance 
was accomplished with approximately the same stored energ}' and less input power. 



DD,r:..1473 

S/N 0101 -807-681 1 



(PAGE 1 ) 



83 



Unclassified 



Security Classification 



A-ai40S 



Jnclassified 



Security Classification 



KEY wo ROS 



Ignition Systems 
Capacitive Discharge 



BOLE WT 



l.rv".,1473 'back: 



FORM 

I NO V 65 

101 -807-682 1 



84 



Unclassified 



Security Classification 



A - 3 I 409 










i'^2, 
-^^i/^ 

? ? 



? -r <5 5 



^339 



^Ppitiyt, jyst^n, 



is 









^0 









Jg 



Thesis 

W6273 

c.l 



133913 

Wi 1 1 iamson 

Ignition system re- 
quirements and their 
application to the de- 
sign of capacitor dis- 
charge ignition systems. 



thesW6273 

'SS.V^'""' requirements and their a 




3 2768 000 98723 4 

n DUDLEY KNOX LIBRARY